Hydroprocessing configuration for low sulfur diesel

ABSTRACT

Methods are provided for producing low sulfur diesel fuels by performing multi-stage hydroprocessing at low pressure on a distillate feed. A feedstock suitable for forming a diesel fuel product is hydrotreated at a hydrogen partial pressure of 500 psig or less in at least two reaction stages. In order to provide improved desulfurization and/or aromatic saturation activity in the final stage, the stages are configured so that the highest hydrogen pressure and/or highest hydrogen purity are delivered to the last hydrotreatment stage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser. No. 61/740,508 filed Dec. 21, 2012, which is herein incorporated by reference in its entirety.

FIELD

This invention provides methods for low pressure multi-stage hydroprocessing of distillate boiling range fractions.

BACKGROUND

The general trend in regulations on fuel products is toward lower contents of sulfur and polyaromatic hydrocarbons (PAH) in the products. The more stringent requirements can pose difficulties for conventional systems, and in particular for conventional systems that operate at low hydrogen partial pressures. Improved reactivity for desulfurization and/or PAH removal can be achieved by using increased reaction temperatures, increased partial pressures, or by processing higher value feedstocks. However, these options require increased processing costs and/or increased hydrogen consumption.

U.S. Pat. No. 6,054,041 describes a method for processing a distillate boiling range feed. A reaction system with two (or more) hydroprocessing stages is used to process a feed to produce a diesel boiling range product. In the hydroprocessing method, fresh hydrogen is added to the final hydroprocessing stage, with the remaining stages being fed from the output effluent from the final hydroprocessing stage. A configuration is described where the compressor for the recycle loop is located prior to the first reaction stage.

SUMMARY OF THE PREFERRED EMBODIMENTS

In an embodiment, a method for producing low sulfur distillate products is provided. The method includes exposing a feedstock having a sulfur content of at least about 1500 wppm and a first content of polyaromatic hydrocarbons to hydrotreating catalyst in the presence of a recycled hydrogen treat gas stream in at least a first reaction stage under first effective hydrotreating conditions to produce an intermediate effluent comprising at least a gas phase fraction and a liquid fraction; separating the liquid fraction from the gas phase fraction, the separated liquid fraction having a sulfur content of at least about 250 wppm; scrubbing the gas phase fraction to remove at least a portion of H₂S, NH₃, or a combination thereof from the gas phase fraction; compressing the scrubbed gas phase fraction; exposing the separated liquid effluent to a second hydrotreating catalyst in the presence of the compressed gas phase fraction and a fresh hydrogen treat gas stream under second effective hydrotreating conditions to produce hydrotreated effluent; and separating the hydrotreated effluent to form at least a diesel boiling range product and a second gas phase fraction, the diesel boiling range product having a sulfur content of 25 wppm or less and a polyaromatic hydrocarbon content of about 7 wt % or less, wherein the recycled hydrogen treat gas stream comprises at least a portion of the second gas phase fraction.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 schematically shows a reaction system for performing a process according to an embodiment of the invention.

FIG. 2 shows predictions for performance based on two different processing configurations, with each configuration showing two different conditions based on modeling data.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In various embodiments, methods are provided for producing low sulfur diesel fuels by performing multi-stage hydroprocessing at low pressure on a distillate feed. A feedstock suitable for forming a diesel fuel product is hydrotreated at a hydrogen partial pressure of 500 psig or less in at least two reaction stages. In order to provide improved desulfurization and/or aromatic saturation activity in the final stage, the stages are configured so that the highest hydrogen pressure and/or highest hydrogen purity are delivered to the last hydrotreatment stage.

When hydroprocessing a sulfur-containing feed, the final sulfur removed from the feed will often contain the hardest (i.e. least reactive) sulfur to remove. Additionally, the final hydroprocessing stage will have a substantial impact on the final PAH concentration of the effluent, as the creation/destruction of PAH compounds is an equilibrium process. Having higher hydrogen pressure and/or hydrogen purity in the final reaction stage can allow for improvements in both desulfurization and reducing PAH content. Having an increased hydrogen concentration in the final reaction stage can reduce the temperature required in the final reaction stage to achieve a desired sulfur content in the effluent. Allowing a reduced temperature is also beneficial for reducing PAH content, as both reduced temperatures and increased hydrogen concentration favor lower amounts of PAH in an effluent.

An increased hydrogen concentration can be achieved in the final hydroprocessing stage of a reaction system based on several configuration features. First, the reaction system can be configured so that all or substantially all (such as at least 90%) of the makeup hydrogen introduced into the reaction system can be introduced into the final reaction stage. Second, a gas-liquid separation can be performed prior to the final reaction stage, so that any contaminants generated during hydroprocessing (such as H₂S or NH₃) are removed from the effluent prior to passing the effluent into the final reaction stage. Additionally, the reaction system can be configured so that the majority of the hydrogen entering a given hydroprocessing stage can be recycled hydrogen. At least one compressor in the recycle loop for providing recycled hydrogen can be located prior to the final hydroprocessing stage, so that there is not another hydroprocessing stage between the compressor and the final hydroprocessing stage. Introducing all or substantially all of the makeup hydrogen into the final stage can help with providing the final reaction stage with a higher hydrogen purity than the other hydroprocessing stages. Locating the compressor prior to the final hydroprocessing stage can allow the hydrogen partial pressure in the final stage to be higher than in any other hydroprocessing stage. The combination of highest hydrogen pressure and highest hydrogen purity in the final stage allows for a lower temperature than earlier stages while still achieving a desired amount of desulfurization in the final stage. This removes the need for having a separate hydrofinishing stage in order to reduce PAH content to a desired level. Instead, the stage that reduces the PAH amounts to a desirable level can also be used to perform more than a minimal amount of desulfurization.

Feedstocks

In an embodiment, a feedstock can have an initial boiling point of at least about 350° F. (177° C.), such as at least about 400° F. (204° C.), or at least about 450° F. (232° C.), or at least about 500° F. (260° C.). Additionally or alternately, the feedstock can have a final boiling point of about 850° F. (454° C.) or less, such as about 800° F. (427° C.) or less, or about 750° F. (399° C.) or less, or about 700° F. (371° C.) or less. Alternatively, the feedstock can be characterized by the boiling point required to boil a specified percentage of the feed. For example, the temperature required to boil at least 5 wt % of a feed is referred to as a “T5” boiling point. Preferably, the hydrocarbon feedstock has a T5 boiling point at least about 350° F. (177° C.), such as at least about 400° F. (204° C.), or at least about 450° F. (232° C.), or at least about 500° F. (260° C.). Preferably, the mineral hydrocarbon feed has a T95 boiling point of about 850° F. (454° C.) or less, such as about 800° F. (427° C.) or less, or about 750° F. (399° C.) or less, or about 700° F. (371° C.) or less. Examples of suitable feeds include various distillate boiling range feeds that are suitable for use as a diesel fuel after hydrodesulfurization.

Mineral feedstreams suitable for use in various embodiments can have a nitrogen content from about 50 wppm to about 6000 wppm nitrogen, preferably about 50 wppm to about 2000 wppm nitrogen, and more preferably about 75 wppm to about 1000 wppm nitrogen. In an embodiment, feedstreams suitable for use herein have a sulfur content from about 100 wppm to about 40,000 wppm sulfur, preferably about 200 wppm to about 30,000 wppm, and more preferably about 350 wppm to about 25,000 wppm.

In various embodiments of the invention, the feed can also include feeds from biocomponent sources, such as vegetable sources or animal sources. The feed can include varying amounts of feedstreams based on biocomponent sources, such as vegetable oils, animal fats, fish oils, algae oils, etc. The feed can include at least 0.1 wt % of feed based on a biocomponent source, or at least 0.5 wt %, or at least 1 wt %, or at least 3 wt %, or at least 10 wt %, or at least 15 wt %. In such embodiments, the feed can include 60 wt % or less of biocomponent, or 50 wt % or less, or 40 wt % or less, or 30 wt % or less. In other embodiments, the amount of co-processing can be small, with a feed that includes at least 0.5 wt % of feedstock based on a biocomponent source, or at least 1 wt %, or at least 2.5 wt %, or at least 5 wt %. In such an embodiment, the feed can include 20 wt % or less of biocomponent based feedstock, or 15 wt % or less, or 10 wt % or less, or 5 wt % or less.

In the discussion below, a biocomponent feedstock refers to a hydrocarbon feedstock derived from a biological raw material component, such as vegetable fats/oils or animal fats/oils (including fish and algae fats/oils). Note that for the purposes of this document, vegetable fats/oils refer generally to any plant based material, and include fat/oils derived from a source such as plants from the genus Jatropha. The vegetable oils and animal fats that can be used in the present invention include any of those which comprise primarily triglycerides and free fatty acids (FFA). The triglycerides and FFAs contain aliphatic hydrocarbon chains in their structure having 8-24 carbons. Other types of feed that are derived from biological raw material components include fatty acid esters, such as fatty acid methyl esters. Examples of biocomponent feedstocks include but are not limited to rapeseed (canola) oil, corn oil, soy oils, castor oil, and palm oil.

Biocomponent based diesel boiling range feedstreams typically have low nitrogen and sulfur content. For example, a biocomponent based feedstream can contain up to about 300 wppm nitrogen. Instead of nitrogen and/or sulfur, the primary heteroatom component in biocomponent based feeds is oxygen. Suitable biocomponent diesel boiling range feedstreams can include up to about 10-12 wt % oxygen.

In this discussion, the term “hydrocarbon” is defined to include molecules that are primarily composed of carbon and hydrogen, but that may also include contaminant atoms typically found in mineral or alternative feedstocks. Thus, unless otherwise specified, hydrocarbons are defined to include compounds that contain atoms such as S, N, or O that are commonly found in refinery feedstocks and/or products.

Reaction System

In an embodiment, the reaction system can include the following features. The feedstock is first treated in a hydrotreatment reactor including a plurality of hydrotreatment stages or beds. The stages in the hydrotreatment reaction system can be operated at a pressure below about 800 psig (5.5 MPag), such as below about 700 psig (4.8 MPa). Preferably, the hydrogen partial pressure in the reaction system can be 500 psig (3.4 Mpag) or less. For example, the hydrogen partial pressure in a stage in the hydrotreatment reactor can be at least about 300 psig (2.1 MPa), or at least about 350 psig (2.4 MPa), or at least about 400 psig (2.8 MPa), or at least about 450 psig (3.1 MPa). Additionally or alternately, the pressure in a stage in the hydrotreatment reactor can be about 700 psig (4.8 MPa) or less, such as about 650 psig (4.5 MPa) or less, or about 600 psig (4.1 MPa) or less, and preferably about 500 psig (3.4 MPag) or less, or 450 psig (3.1 MPag) or less. The hydrotreatment stages can reduce the sulfur content of the feed to a suitable level. For example, the sulfur content can be reduced to about 100 wppm sulfur or less, such as 50 wppm or less, or about 25 wppm or less, or about 10 wppm or less. Preferably, the sulfur content of the effluent from stage prior to the final hydroprocessing stage can be at least about 200 wppm, such as at least about 300 wppm, or at least about 500 wppm. Optionally but preferably, the final hydroprocessing stage can be in a separate reactor from the prior stage.

The other reaction conditions in a hydrotreatment stage can be conditions suitable for reducing the sulfur content of the feedstream. The reaction conditions can include an LHSV of 0.3 to 5.0 hr⁻¹, a treat gas containing at least about 80% hydrogen (remainder inert gas), and a temperature of from about 500° F. (260° C.) to about 800° F. (427° C.). Preferably, the reaction conditions include an LHSV of from about 0.5 to about 1.5 hr⁻¹ and a temperature of from about 700° F. (371° C.) to about 750° F. (399° C.).

The catalyst in a hydrotreatment stage can be a conventional hydrotreating catalyst, such as a catalyst composed of a Group VIB metal (Group 6 of IUPAC periodic table) and/or a Group VIII metal (Groups 8-10 of IUPAC periodic table) on a support. Suitable metals include cobalt, nickel, molybdenum, tungsten, or combinations thereof. Preferred combinations of metals include nickel and molybdenum or cobalt and molybdenum or nickel, cobalt, and molybdenum. Suitable supports include silica, silica-alumina, alumina, and titania.

In an embodiment, the amount of treat gas delivered to the hydrotreatment stage can be based on the consumption of hydrogen in the stage. The treat gas rate for a hydrotreatment stage can be from about two to about five times the amount of hydrogen consumed per barrel of fresh feed in the stage. A typical hydrotreatment stage can consume from about 50 SCF/B (8.4 m³/m³) to about 1000 SCF/B (168.5 m³/m³) of hydrogen, depending on various factors including the nature of the feed being hydrotreated. Thus, the treat gas rate can be from about 100 SCF/B (16.9 m³/m³) to about 5000 SCF/B (842 m³/m³). Preferably, the treat gas rate can be at least about two times the amount of hydrogen consumed, such as from about four to about five time the amount of hydrogen consumed. Note that the above treat gas rates refer to the rate of hydrogen flow. If hydrogen is delivered as part of a gas stream having less than 100% hydrogen, the treat gas rate for the overall gas stream can be proportionally higher.

The effluent from at least the stage prior to the final stage is preferably passed into a separator. The separator allows for separation of liquid effluent from contaminant gases formed during hydrotreatment, such as hydrogen sulfide or ammonia. Gas phase hydrocarbons (such as light ends) produced in the prior reactors can also be removed.

Configuration Example

FIG. 1 shows an example of a suitable reaction configuration. In FIG. 1, a diesel boiling range feedstock 105 is introduced into a first hydrotreatment reaction stage and/or reactor 110 along with a recycled hydrogen stream 185. Because there are only two reaction stages and/or reactors, the first reaction stage is also the stage prior to the last hydrotreatment stage in this configuration. The feedstock 105 is exposed to one or more hydrotreating catalysts in the reaction stage 110 to generate a (partially) desulfurized effluent 115. The effluent 115 is passed into a gas-liquid separator 120 to form a liquid effluent 125 and a gas phase effluent 122. The gas phase effluent 122 is then passed into a scrubber 130 for removal of H₂S and NH₃, which are contaminant gases formed from removal of sulfur and nitrogen from a diesel boiling range feedstock. This reduces the contaminant gas content that will eventually be passed into second hydrotreatment reaction stage and/or reactor 150. The scrubbed gas phase effluent 132 is then passed into a compressor 140 to increase the pressure of the recycle stream. The compressor can replace any pressure loss that occurred as the gas flow passes through the reaction stages or separators. The compressed gas phase effluent 142 is then introduced into hydrotreatment reactor and/or reaction stage 150. The liquid effluent 125 is also passed into reaction stage 150, along with a fresh makeup stream of hydrogen 152. The liquid effluent is then exposed to one or more hydrotreatment catalysts in the second hydrotreatment stage and/or reactor to generate a second hydrotreated effluent 155. The second hydrotreated effluent can have a sulfur content of 100 wppm or less, such as 50 wppm or less, or 25 wppm or less, or 15 wppm or less, or 10 wppm or less. The second hydrotreated effluent also has a polyaromatic hydrocarbon content of less than about 7 wt %, such as less than about 6.75 wt %, or less than about 6.5 wt %. Additionally or alternately, the polyaromatic hydrocarbon content in the second hydrotreated effluent can be dependent on the amount of polyaromatic hydrocarbons in the feed to the initial reaction stage.

The second hydrotreated effluent 155 can then be passed through one or more separation stages 160 to separate a second liquid phase effluent 165 from gas phase effluent that can be used as a recycle stream 185. A portion of the gas phase effluent can be a purge 187 to allow release of light hydrocarbons, thus improving the overall purity of the overall recycle loop, including recycle stream 185. The second liquid phase effluent can optionally be a diesel boiling range effluent, or fractionation 170 may be used to separate out any naphtha boiling range products 172 from a desired diesel boiling range product 175. Any naphtha boiling range products 172 may be present due to the fact that hydrotreatment stages often result in some conversion of a feed to lower boiling products.

Example 1 Predicted Performance for Reaction System Configuration

An empirical model for predicting hydroprocessing performance was used to compare the performance of the configuration in FIG. 1 with the performance of a conventional reaction system configuration where the compressor for the recycle loop is prior to the first hydrotreatment stage, and where makeup hydrogen is also introduced into the first reaction stage. In the conventional configuration, no separation is performed between the first and second reaction stages.

FIG. 1 includes various values for input and output flows from some elements in FIG. 1. In addition to the flow rates for various flows, the sulfur (0.52 wt %) and nitrogen (87 wppm) contents for the input feed are shown, along the sulfur (9.8 wppm) and nitrogen (3.5 wppm) contents for the final diesel product. The values shown in FIG. 1 are for operation of the system with a target of achieving 10 wppm or less of sulfur.

For comparison, FIG. 2 shows a comparison between four different sets of conditions that were modeled using the empirical model. In FIG. 2, R1 and R2 refer to the first and second hydrotreatment stages, respectively. WABT refers to weighted average bed temperature. Cat Distribution R1/R2 refers to how much catalyst is present in each reactor. For the data shown in FIG. 2, the same catalyst (or catalyst system) was used in both hydrotreatment reactors, with 25 wt % of the catalyst in the first hydroprocessing stage. Diesel 2R+ aromatics refers to the content of polyaromatic hydrocarbons in the final diesel product.

FIG. 2 shows predictions for performance based on four configurations/conditions. The first two columns represent predictions for a conventional hydrotreatment system as described above, which does not include a separation stage between the first and second hydrotreatment reactors or stages and where the makeup hydrogen is introduced into the first hydrotreatment reactor. Column 1 shows predicted results from simulations where the second reactor was constrained to have a weighted average bed temperature of about 650° F. Column 2 shows predicted results where the temperature of the reactors are set so that a sulfur content of 10 wppm is achieved in the final product. Columns 3 and 4 are based on the reaction configuration shown in FIG. 1. Column 3 has the same constraint as Column 1 (reactors are 650° F.) while Column 4 has the same constraint as Column 2 (10 wppm sulfur in final product).

As shown in FIG. 2, the configuration shown in FIG. 1 can achieve a final product sulfur content of 10 wppm or less at a temperature that is 15° F. cooler than the conventional configuration. (It is noted that both reactor 1 and reactor 2 for the conventional configuration have a higher temperature in order to meet the desired sulfur target.) Due in part to the cooler temperature in the final reaction stage, as well as the reduced H₂S and/or NH₃ content in the treat gas, the content of polyaromatic hydrocarbons lower by more than 2 wt % in Column 4 versus Column 2.

It is also noted that the sulfur and nitrogen contents of the effluent that is passed into the final hydrotreatment stage is higher for the reaction configuration in FIG. 1. For example, in Column 1, the sulfur content of the effluent passed into the final hydrotreatment stage is 220 wppm, while in Column 2 the sulfur content into the final hydrotreatment stage is 150 wppm. By contrast, the sulfur content into the final hydrotreating stage in Columns 3 and 4 is about 270 wppm or 280 wppm. Thus, even though the final hydrotreatment stage for the Configuration in FIG. 1 performs a larger amount of desulfurization, the configuration in FIG. 1 also produces a lower PAH content.

Additional Embodiments Embodiment 1

A method for producing low sulfur distillate products, comprising: exposing a feedstock having a sulfur content of at least about 1500 wppm and a first content of polyaromatic hydrocarbons to hydrotreating catalyst in the presence of a recycled hydrogen treat gas stream in at least a first reaction stage under first effective hydrotreating conditions to produce an intermediate effluent comprising at least a gas phase fraction and a liquid fraction; separating the liquid fraction from the gas phase fraction, the separated liquid fraction having a sulfur content of at least about 250 wppm; scrubbing the gas phase fraction to remove at least a portion of H₂S, NH₃, or a combination thereof from the gas phase fraction; compressing the scrubbed gas phase fraction; exposing the separated liquid effluent to a second hydrotreating catalyst in the presence of the compressed gas phase fraction and a fresh hydrogen treat gas stream under second effective hydrotreating conditions to produce hydrotreated effluent; and separating the hydrotreated effluent to form at least a diesel boiling range product and a second gas phase fraction, the diesel boiling range product having a sulfur content of 25 wppm or less and a polyaromatic hydrocarbon content of about 7 wt % or less, wherein the recycled hydrogen treat gas stream comprises at least a portion of the second gas phase fraction.

Embodiment 2

The method of Embodiment 1, wherein the effective hydrotreating conditions comprise a hydrogen partial pressure from about 300 psig (2.1 MPa) to about 800 psig (5.5 MPa), a temperature of from about 500° F. (260° C.) to about 800° F. (427° C.), and a space velocity of from about 0.3 hr⁻¹ to about 5.0 hr⁻¹.

Embodiment 3

The method of any of the above embodiments, wherein the effective hydrotreating conditions include a treat gas rate that provides an amount of hydrogen from about two times to about five times the hydrogen consumed during the hydrotreating.

Embodiment 4

The method of any of the above embodiments, wherein the sulfur content of the diesel boiling range product is about 10 wppm or less.

Embodiment 5

The method of any of the above embodiments, wherein the sulfur content of the separated liquid fraction is at least about 500 wppm.

Embodiment 6

The method of any of the above embodiments, wherein the fresh hydrogen treat gas stream comprises at least about 80 wt % of hydrogen.

Embodiment 7

The method of any of the above embodiments, wherein the at least a first reaction stage comprises a plurality of reaction stages, the intermediate effluent comprising an effluent produced after exposure to hydrotreating catalyst in each of the plurality of reaction stages.

Embodiment 8

The method of Embodiment 7, wherein the second hydrotreating catalyst is the same as a hydrotreating catalyst in at least one of the plurality of reaction stages.

Embodiment 9

The method of any of the above embodiments, wherein separating the liquid fraction from the gas phase fraction comprises performing a flash separation on the intermediate effluent.

Embodiment 10

The method of any of the above embodiments, wherein the diesel boiling range product has a polyaromatic hydrocarbon content of about 6.5 wt % or less.

Embodiment 11

The method of any of the above embodiments, wherein the hydrogen partial pressure is about 500 psig (3.4 MPag) or less. 

What is claimed is:
 1. A method for producing low sulfur distillate products, comprising: exposing a feedstock having a sulfur content of at least about 1500 wppm and a first content of polyaromatic hydrocarbons to hydrotreating catalyst in the presence of a recycled hydrogen treat gas stream in at least a first reaction stage under first effective hydrotreating conditions to produce an intermediate effluent comprising at least a gas phase fraction and a liquid fraction; separating the liquid fraction from the gas phase fraction, the separated liquid fraction having a sulfur content of at least about 250 wppm; scrubbing the gas phase fraction to remove at least a portion of H₂S, NH₃, or a combination thereof from the gas phase fraction; compressing the scrubbed gas phase fraction; exposing the separated liquid effluent to a second hydrotreating catalyst in the presence of the compressed gas phase fraction and a fresh hydrogen treat gas stream under second effective hydrotreating conditions to produce hydrotreated effluent; and separating the hydrotreated effluent to form at least a diesel boiling range product and a second gas phase fraction, the diesel boiling range product having a sulfur content of 25 wppm or less and a polyaromatic hydrocarbon content of about 7 wt % or less, wherein the recycled hydrogen treat gas stream comprises at least a portion of the second gas phase fraction.
 2. The method of claim 1, wherein the effective hydrotreating conditions comprise a hydrogen partial pressure from about 300 psig (2.1 MPa) to about 800 psig (5.5 MPa), a temperature of from about 500° F. (260° C.) to about 800° F. (427° C.), and a space velocity of from about 0.3 hr⁻¹ to about 5.0 hr⁻¹.
 3. The method of claim 1, wherein the effective hydrotreating conditions include a treat gas rate that provides an amount of hydrogen from about two times to about five times the hydrogen consumed during the hydrotreating.
 4. The method of claim 1, wherein the sulfur content of the diesel boiling range product is about 10 wppm or less.
 5. The method of claim 1, wherein the sulfur content of the separated liquid fraction is at least about 500 wppm.
 6. The method of claim 1, wherein the fresh hydrogen treat gas stream comprises at least about 80 wt % of hydrogen.
 7. The method of claim 1, wherein the at least a first reaction stage comprises a plurality of reaction stages, the intermediate effluent comprising an effluent produced after exposure to hydrotreating catalyst in each of the plurality of reaction stages.
 8. The method of claim 7, wherein the second hydrotreating catalyst is the same as a hydrotreating catalyst in at least one of the plurality of reaction stages.
 9. The method of claim 1, wherein separating the liquid fraction from the gas phase fraction comprises performing a flash separation on the intermediate effluent.
 10. The method of claim 1, wherein the diesel boiling range product has a polyaromatic hydrocarbon content of about 6.5 wt % or less.
 11. The method of claim 1, wherein the hydrogen partial pressure is about 500 psig (3.4 MPag) or less. 